Multi-site simulcast radio communication systems are well known in the art for providing wide area communications. More recently, digital simulcast radio communications systems have been developed. In such systems, digital information is formatted into information frames. Each information frame is made up of a plurality of information packets that are transported through the system infrastructure.
All simulcasted information packets in many such digital simulcast radio communication systems are processed through a comparator. The comparator receives inputs from multiple signal sources and selects an input signal source based on predetermined criteria of signal quality. The comparator then assigns a launch time to each information packet received from the selected input signal source, and transmits the information packet over an infrastructure link to at least one of a plurality of base stations, where the information packet is temporarily stored in a buffer. At the assigned launch time, error correcting information is added to the information packet and the resulting data packet is transmitted by the base stations. (The use of error correcting information to improve the reliability of transmitted information on a radio channel is well known in the art and no further elaboration need be provided here.)
For any given information packet, adding the error correcting information to the information packet prior to transmission over the radio channel typically results in a longer required transmission time on the radio channel than over the infrastructure link. For example, a particular information packet can take 20 milliseconds to transmit over the infrastructure link from the comparator to the base station, and 45 milliseconds to transmit on the radio channel. To synchronize the different transmission rates, the comparator therefore determines the launch time for an information packet as a function of both the transmission time and launch time of previous information packets transmitted on the radio channel.
Operational conditions can occur, however, that cause the launch time assignments of the comparator to fall behind present time. When the base station encounters an information packet with such a launch time assignment, the base station is unable to transmit the information packet as the launch time has already passed relative to the present time. In addition, because launch times for subsequent information packets are determined from previously assigned launch times, subsequent information packets will not be transmitted either. As a result, the user may experience a considerable loss of information.
Alternatively, operational conditions can occur where launch time assignments by the comparator are considerably ahead of present time. This causes the base stations to transmit the information packets at a significantly later time, resulting in a delay, that is particularly noticeable when audio information is transmitted.
An example of when such operational conditions can occur is when the comparator switches input signal sources during a call. At such an instance, it is probable that information packets from a previous source and a new source are not synchronized. This situation may cause the comparator to assign launch times that cause loss of information and delay of information.
Accordingly, a method of transmitting information packets from the comparator to a buffer that avoids, or at least minimizes, some of these problems is desired.